Prominent cirrus clouds over Lisbon.
Cirrus cloud is a type of cloud.
The word cirrus orginates from Latin, meaning 'whisp of hair'.
Cirrus is composed of ice crystals and shaped like hairlike filaments. It is formed at an altitudes above 16,500 feet. If there are many cirrus clouds in the sky it may be a sign that a frontal system is approaching. Cirrus clouds can also be the remnants from a thunderstorm.